1. Field of the Invention
This invention relates generally to phase tracking devices that dynamically track the phase or frequency of an input signal within a given bandwidth as in the case of a phase lock loop. More particularly, the invention relates to a phase tracking device that maintains a constant duty cycle for the half cycles within each complete cycle despite rapid changes in the input frequency being tracked.
2. Description of the Prior Art
Various phase lock loops are known. A typical phase lock loop utilizes a voltage controllable oscillator (VC0) whose output frequency is controlled by a control voltage. A phase detector compares the phase difference between the signal from the VCO and the signal being tracked and provides an output signal proportional to the phases of the two signals. The output signal from the phase detector is applied to the frequency controlling input of the VCO and adjusts the frequency of the VCO until the phase difference between the two signals is zero or another desired value. The VCO will then track any changes in the frequency of the input signal. However, if the input frequency has a step change, the output of the phase lock loop will slew over to the new frequency with a continuously changing frequency until the loop settles. The continuously changing frequency of the phase lock loop during the slewing period will result in a waveformat the output of the phase lock loop that does not have a 50% duty cycle. For example, if the phase lock loop is being utilized as a demodulator, it is important that the phase lock loop signal applied to the phase detector and compared with the input signal have an exact 50% duty cycle. If the first half cycle is not equal to the second half cycle, then the demodulated output will contain rectified information, such as a DC offset, in addition to the desired AC signal information.